MdeModulePkg/IScsiDxe: Set ExitBootServiceEvent to NULL after close it.

v2:
* Refine the commit log.

There are two place to close the ISCSI ExitBootServiceEvent:
#1.IScsiOnExitBootService(), which is the callback function of
   ExitBootServiceEvent.
#2.IScsiCleanDriverData(), which will be invoked by ISCSI driver
   binding stop().

So, the ExitBootServiceEvent will be closed and freed when exit boot
server is triggered. But it may be closed and freed again in ISCSI driver
binding stop(), which will result in the issue recorded at
https://bugzilla.tianocore.org/show_bug.cgi?id=742.

This patch is to resolve the issue.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
(cherry picked from commit d321598)

MdeModulePkg/Universal/Network/IScsiDxe/IScsiMisc.c

@@ -1,7 +1,7 @@
 /** @file
   Miscellaneous routines for iSCSI driver.
 
-Copyright (c) 2004 - 2016, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2004 - 2018, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
 which accompanies this distribution.  The full text of the license may be found at
@@ -624,9 +624,10 @@ IScsiCleanDriverData (
   }
 
 EXIT:
-
-  gBS->CloseEvent (Private->ExitBootServiceEvent);
-
+  if (Private->ExitBootServiceEvent != NULL) {
+    gBS->CloseEvent (Private->ExitBootServiceEvent);
+  }
+
   FreePool (Private);
   return Status;
 }
@@ -872,7 +873,9 @@ IScsiOnExitBootService (
   ISCSI_DRIVER_DATA *Private;
 
   Private = (ISCSI_DRIVER_DATA *) Context;
+
   gBS->CloseEvent (Private->ExitBootServiceEvent);
+  Private->ExitBootServiceEvent = NULL;
 
   IScsiSessionAbort (&Private->Session);
 }